1. Field of the Invention
The present invention relates to a method for producing thin films of silicon nitride. More specifically, the present invention relates to a method of producing a thin film of silicon ceramic-like nitride on an object by subjecting a vapor of a cyclic silicon-nitrogen precursor, such as 1,1,3,3,5,5-hexamethylcyclotrisilazane, with hydrogen alone or a source of hydrogen and nitrogen, such as ammonia. These thin films do not require the usual post-deposition pyrolysis at 800.degree. C. or higher, and are useful as passivation coatings on objects, such as semiconductor inter metal dielectric layers.
2. Description of the Related Art
Plasma enhanced chemical vapor deposition (PECVD) of silicon nitride (SiNxHy, where x is between about 0.8 and 1.5 and y is between about 0.4 and 1.2) are used as final passivation layers in microelectronic circuits. Silicon nitride is preferred due to its impermeability to water and to sodium ion diffusion as well as mechanical toughness and high dielectric constant.
In the past, typical methods for film production involve the plasma enhanced reaction of silane (SiH.sub.4) with either nitrogen (N.sub.2) or ammonia (NH.sub.3). The difficulties encountered in the control of SiNxHy stoichiometry along with the pyrophoric nature of silane has lead to the investigation of organosilicon monomers as sources of silicon for SiNxHy thin film formation.
In U.S. Pat. No. 4,158,717, N. H. Nelson discloses a preparation of a silicon nitride film by the plasma discharge of a vapor of azidotrimethylsilane. Temperatures between 100.degree.-400.degree. C. and a pressure of 0.15 to 0.20 Torr were used.
M. R. Gulett et al., in U.S. Pat. No. 4,330,569 disclose a method of conditioning a nitride surface.
J. Janca et al., in Acta Phys. Slov., Vol. 33 (#3), pp. 187-193, published in 1983 disclose the preparation of thin silicon nitride films which are deposited in the RF discharge plasma of 1.5 MHz of vaporizable linear hexamethyldisilazane [(CH.sub.3).sub.3 Si].sub.2 NH, or azidotrimethylsilane (CH.sub.3).sub.3 SiN.sub.3 in the presence of nitrogen.
V. I. Belyi et al., in Mikroelectronica (Akad.Nauk SSSR), Vol. 15, p. 146-149 (1986) [see Soviet Microelectronics, Vol. 15, No. 2, p. 91-94 (1986)], disclose the plasma polymerization of hexamethylcyclotrisilazane at 0.1 W/cm.sup.2 (RF) in ammonia. The film produced showed large Si-H and N-H peaks by infrared spectral analysis, as well as substantial Si-C and C-H absorptions.
D. V. Tsu et al., in Physical Review B:, Vol. 33 (#10), pp. 7069 to 7076, published in 1986, disclose the preparation of silicon nitride thin films by remote plasma-enhanced chemical vapor deposition using gas mixtures of silane and either nitrogen or ammonia.
B. Arkles in the Journal of the Electrochemical Society, pp. 233-234, published in January, 1986, discloses that silicon nitride is formed from cyclic and linear organosilazane prepolymers as single components. The materials are pyrolyzed at temperatures of 350.degree. C. or greater. Complete conversion to ceramic occurs at temperatures exceeding 700.degree. C.
A. M. Wrobel in the Journal of Macromolecular Science-Chemistry, Vol. A15 (#2), pp. 197-213, published in 1981, disclose the polymerization of hexamethyltrisilazane as a single component in a microwave plasma at 2.45 gigahertz (GHz). The polymerizations to provide thin polysilazane films were conducted at substrate temperatures of 25.degree., 200.degree., 400.degree., 600.degree. and 800.degree. C.
A. M. Wrobel et al., disclose in Polymer, Vol. 17, pp. 673-677 and 678-684, published in 1986, the prepartion of thin polysilazane films by the glow discharge polymerization of hexamethylcyclotrisilazane as a single component at 20 kHz.
A. M. Wrobel et al., in Plasma Polymerization (M. Shen and A. T. Bell, eds.) American Chemical Society Symposium Services, No. 108, ACS, Washington, D.C., pp. 237-249, published in 1979, disclose the plasma polymerization of hexamethylcyclotrisilazane in a glow discharge at 20 kilohertz (kHz). A combination of the monomer and ammonia at 0.3 Torr were used in one example. Considerable organic character is present. Pyrolysis of the layer at 600.degree. and 800.degree. C. showed a loss of Si-C bonds, formation of methane, and increasing inorganic ceramic character.
M. Kryszewski et al., in Plasma Polymerization (M. Shen and A. T. Bell, eds.) American Chemical Society Symposium Series, No. 108, ACS, Washington, D.C., Chapter 13, pp. 219-235, published in 1979, disclose the plasma polymerization of hexamethylcyclotrisilazane vapor under glow discharge conditions of 20 kilohertz (kHz). Upon pyrolysis of the thin films at a RF value of 13.5 MHz in a glow discharge, cyclic SiN compounds were evolved.
J. Tyczkowski et al., in Thin Solid Films, Vol. 55, pp 253-259, published in 1978, disclose the production of polysilazane films by the hexamethylcyclotrisilazane as a single component in a glow discharge at 20 kHz.
M. Gazicki et al., in Plasma Chemistry and Plasma Processing, Vol. 3, (No. 3), pp. 279-327, published in 1983, disclose the glow discharge plasma polymerization of hexamethylcyclotrisilazane under glow discharge conditions.
M. Gazicki et al., in the Journal of Applied Polymer Science, Vol. 21, pp. 2031-2019, published in 1977, disclose the glow discharge polymerization at 20 kHz of hexamethylcyclotrisilazane as a single component to produce polysilazane as a thin film.
N. Voke et al. Materials Research Society Symposia Proceedings (Symposium held in Palo Alto, Calif., U.S.A., Apr. 15-18, 1986), Vol. 68, pp. 175-181, published in Pittsburgh, Pa. in 1986, disclose the plasma enhanced chemical vapor deposition of silicon-nitride thin films under different experimental conditions using silane and ammonia.
W. Verbeek in U.S. Pat. No. 3,853,567 and G. W. Winter, et al., in U.S. Pat. No. 3,892,583 disclose a method of making a shaped article of silicon carbide and nitride of silicon carbide and nitride by decomposing a silazane at about 200.degree. to 800.degree. C.
T. H. Hirai et al., in the Journal of Materials Science, Vol. 16, pg. 17-29 and also on pp. 2877-2882, published in 1981 disclose the deposition of CVD-S.sub.3 N.sub.4 layers including carbon.
J. J. Nicki et al., in the Journal of Less-Common Metals, Vol. 37, pg. 317-329, published in 1974, disclose the CVD of layers of Si-C and Si-C-N.
C. L. Beaty in "Silicon Nitride and Silicon Carbide from Organometallic and Vapor Precursors", Chapter 3 in Ultrastructure Processing of Ceramics, Glasses and Composites, edited by L. L. Hench, John Wiley and Sons, New York, N.Y., published in 1984 disclose a number of methods of preparation of silicon nitride.
E. J. Charlson et al., in "Electrical Properties of Glow Discharge Polymerized Films", in Organic Coatings and Applied Polymer Science Proceedings, Vol. 47, pg. 146-150, American Chemical Society, Washington, D.C., published in 1982 disclose some methods of preparing polymerized Si-N films. S. K. Varshney at al., "Surface Structure and Properties of R.F. Plasma Polymerized Hexamethyldisilazane", in the same volume pg. 151-153 disclose the properties of glow discharge produced polymer films from hexamethyldisilazane.
S. K. Varshney et al., "Plasma Polymerization of Silanes", in Organic Coatings and Applied Polymer Sciences Proceedings, Vol. 46, pg. 127-133, American Chemical Society, Washington, D.C. 20036 disclose the results of plasma polymerization of silanes.
K. S. Mazdiyasni et al., in The Journal of the American Ceramic Society, Vol. 61, Nos. 11-12, pg. 504-508, published in 1978 disclose the characterization of organosilicon-infiltrated porous reaction sintered Si.sub.3 N.sub.4.
A. M. Wrobel et al., Journal of Macromolecular Science-Chemistry, Vol. A12, No. 7, pg. 1041-1054, published in 1978 disclose the effect of glow discharge conditions or structure and thermal properties of polysilazane thin films. The films were produced by glow discharge polymerization of hexamethylcyclotrisilazane with various gases: argon, nitrogen, hydrogen and ammonia. The polymer films produced with hydrogen and ammonia under these conditions had considerable organic carbon, (C-H stretches) content. A post deposition pyrolysis of the film is described to obtain specific physical properties.
If the cyclic intermediate is polymerized normally, as in a normal plasma, a porous silicon nitride thin film having --CH.sub.3 groups is obtained. The film is porous to alkali ions and is not dense because the polymeric molecules are not closely packed. The film is not a good chemical barrier because of the porosity. To obtain the desired porosity, it is necessary to pyrolyze the film to 1000.degree. C. or higher to form silicon and carbon radicals which crosslink and produce a film having the desired high density (d=1.8 to 2.1) and low porosity. These temperatures normally damage or destroy other films or substrates present in microelectronic devices, and are to be avoided if at all possible.
A major problem encountered in most of the methods of the above cited art is the use of SiH.sub.4 as a silicon precursor. SiH.sub.4 is a dangerous, pyrophoric material to use under laboratory or industrial conditions. None of the above articles or patents individually or collectively in any fashion teach, disclose or suggest the present invention. It is therefore very useful to have a PECVD process to deposit a thin layer of silicon nitride having a minimum of carbon, oxygen, and/or hydrogen on a substrate under mild reaction and substrate conditions, and without the need for post-deposition pyrolysis to reduce the organic nature of the films to form a stable ceramic film.